Fiber optic cables are used for high-speed communication and data transmission, often over long distances. At present, long haul fiber optic transmission networks may extend for distances as great as 4000 km, and greater distances are possible. In a fiber optic cable, an electrical signal from a communications signal source (e.g., telephone or data modem) is modulated onto a light wave and transported via a fiber optic connection to a receiver, where the electrical signal is recovered through a demodulation process. A critical factor in the implementation of fiber optic technology is the attenuation or signal loss characteristic of the fiber optic cable used as the transmission medium. Signals transmitted via a fiber optic channel attenuate over long distances, and at some distance from the signal source reach a sufficiently low level as to require amplification by a repeater inserted in the fiber channel. For reasons including the loss characteristics of fiber optic cable and other practical considerations, repeater stations on long haul fiber optic transmission lines may typically be located at spans of 80 km to 160 km apart.
For a large scale fiber network, optical fiber occasionally suffers interruptions or faults, for a variety of reasons. For example, fiber optic cables may be cut by accident; aging of fiber optic cables can result in reduced transmissivity, causing the optical signal to weaken; and kinks in fiber optic cable can diminish or interrupt optical signals. In order to repair a fault in a long haul transmission line, the fault must be located, and a field team sent to the fault location. Due to the large distances of the long haul fiber optic networks, a field repair the team may need to work for hours and days simply to locate the fault before they can repair the line. Instruments using optical time domain reflectrometry (OTDR) are effective in locating faults in fiber optic cable. However, due to the nature of long haul fiber systems, in which an amplifier will block any reflection, OTDR can only work for one span, while a long haul system may have as many as twenty spans. Furthermore, the loss of signal caused by a fault may trigger an automatic shutdown of the amplifiers closest to the fault, thereby preventing the transmission of signals, including OTDR signals, through the affected amplifiers. Thus, simply locating a fault in a long haul transmission network may require field teams to travel to multiple repeater locations, where each repeater location may be as far as 160 km from the next repeater station. Furthermore, faults in long haul fiber optic systems may be underground, and thus inaccessible to visual inspection. OTDR may be employed from repeater station amplifiers at both ends of the span containing the fault, in order to locate the fault more precisely than from a single OTDR, and thus to minimize the cost of locating, repairing and/or splicing the fault. However, such a technique necessarily requires more OTDR, at more locations.